Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Abstract

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

Oxidative Stress Response and Metal Transport in Roots of Macleaya cordata Exposed to Lead and Zinc

“…When plants are subjected to heavy metal stress, the elongation growth of their roots is firstly inhibited, followed by their shoot height and dry weight [ 4 ]. In this research, the Pb treatment had no significant effect on the shoot height of M. cordata , while 100 and 200 μmol·L −1 Zn caused a significant decrease in the shoot height.…”

“…Zn-induced ROS were produced and antioxidant enzymes decreased in the roots of the hyperaccumulating ecotype of S. alfredii [ 13 ]. It was also reported that the translocation of heavy metals from the roots to shoots was a detoxification pathway to reduce the stress toxicity to the plant roots [ 4 ].…”

“…Plants have evolved the mechanisms to tolerate and accumulate heavy metal in the process of adapting to their environment. Firstly, plants can control their heavy metal uptake and transport through the transporters located in their plasma membrane or they can sequester the heavy metals in vacuoles by tonoplast transporters [ 1 , 4 ]. Secondly, plants can induce the synthesis of specific organic compounds, including organic acids, amino acids, phytochelatins and metallothioneins, to directly bind with excess metal ions in cells or export them to the apoplast to act as barriers against heavy metals, thereby protecting the cell protoplasm [ 5 ].…”

“…Pb is one of the most toxic environmental pollutants for plants, and Zn is an essential micronutrient associated with proteins and enzymes; however, excess Zn, like Pb, can interfere with numerous biochemical and physiological processes, including metal uptake and transport in plants [ 4 ]. The toxicities of Pb and excess Zn are mediated by the formation of reactive oxygen species (ROS), such as superoxide anions (O 2 − ) and hydrogen peroxide (H 2 O 2 ).…”

Changes in Growth and Heavy Metal and Phenolic Compound Accumulation in Buddleja cordata Cell Suspension Culture under Cu, Fe, Mn, and Zn Enrichment

“…Together, these results indicate a high consumption of the sugar substrate required for growth. At the cellular level, plants can feature a range of detoxification mechanisms that activate to prevent the accumulation of toxic HM concentrations within the cell, such as the involvement of the plasma membrane in reducing the uptake of HM; chelation in the cytosol by ligands, resulting in complex formation; or the transport of HM/HM complexes to vacuoles [ 21 ]. In this study, the plasma membranes of the B. cordata cells, as a detoxification mechanism, may have reduced the uptake of Cu and Mn since HM accumulation greatly increased under the first concentration tested, followed by a slight increase that plateaued as the concentration increased.…”